Wireless communication systems of today have increased demands on supporting higher peak rates and capacity. One of the most popular techniques for achieving this is the utilization of multiple antennas at the transmitter and/or receiver side. How and for which purpose these antennas are utilized differs based among other things on operating channel conditions. For instance, multiple antennas can be utilized to transmit different data streams on the same time/frequency resource in case the Signal to Interference and Noise Ratio (SINR) is high enough and the channels between the different antennas are uncorrelated enough. Another possible utilization of multiple antennas is to transmit different redundant versions of the same information e.g. as in done in the Alamouti scheme, and thereby increasing diversity.
Based on the type of information available at the transmitter, two generic modes of Multiple-Input Multiple-Output (MIMO) operation, which is a transmission scheme between a transmitter and a receiver both equipped with multiple antennas, can be distinguished:
Open Loop (OL) Transmission
OL-MIMO refers to the case where the Precoding Matrix Information (PMI) can not be obtained at the transmitter, or can be obtained with a very low accuracy or short life, e.g. in case of User Equipments (UEs) moving at high speed. OL-MIMO is also referred to as channel independent transmission.
Closed Loop (CL) Transmission
CL-MIMO refers to the case where the transmitter has knowledge of the PMI and Rank Indicator (RI) so that it can choose the transmission parameters in a way that suits the operating channel conditions and further improve the system's performance. In addition to Closed Loop Spatial Multiplexing (CLSM), CL-MIMO also allows utilizing the rank 1 Tx diversity mode, i.e. based on Alamouti coding (e.g. in case the obtained PMI/RI information expired or has a low accuracy). CL-MIMO is also referred to as channel dependent transmission.
In Long Term Evolution (LTE) systems/networks, both CL-MIMO and OL-MIMO will be utilized and configured at a Radio Resource Control (RRC)-level. As such, a switch between CL and OL is not very flexible as it requires a reconfiguration of the UE's connection. On the other hand, if a UE is in CL-mode, it is flexible and straightforward to choose between the different transmissions schemes within i.e. Spatial Multiplexing (SM), precoding-based or Tx diversity. The UE knows which scheme the Base Station (BS) is utilizing by simply examining signals from the BS e.g. the downlink assignment which is obtained from the Downlink Control Information (DCI) in LTE.
In order to utilize CL MIMO, Channel State Information (CSI) of the downlink channel has to be reported from the UE to the BS (eNodeB, eNB). The CSI consists of any combination of Channel Quality Indicator (CQI), PMI and/or RI. In an LTE system/network, the feedback of this information from the UE to the BS can occur on two different channels with two different configurations.
1. Periodic CSI reporting on Physical Uplink Control Channel (PUCCH) which is the LTE uplink physical channel carrying uplink control information including Channel Quality Indicators (CQI), Hybrid Automatic Retransmission reQuest (HARQ) ACKnowledgment/Negative ACKnowledgment (ACK/NACK) and uplink scheduling requests.
The CSI report, periodically reported as mentioned above, typically consists of wideband PMI measures (mean value).
2. Aperiodic CSI reporting on Physical Uplink Shared Channel (PUSCH) which is the LTE uplink physical channel carrying scheduled data traffic, and control signalling if some is required to be transmitted in the same sub-frame.
The CSI report may consist of either wideband PMI with sub-band CQI, or sub-band PMI with wideband CQI.
By wideband PMI, as mentioned above, it is normally meant that a measurement is obtained by averaging over the whole band. This is as opposed to a sub-band measurement where the measurement has a finer granularity and is obtained over a specific part of the bandwidth. The trade-off is of course complexity vs accuracy.
The main benefit of utilizing CL transmissions is the utilization of detailed channel knowledge, e.g. PMI, RI, to better suit the transmission parameters to the operating channel conditions. However, it is known that errors and/or delays in the reported CSI would lead to detrimental effects to the system's performance as evidenced. However, such a problem has not been thoroughly tackled from a multi-antenna closed loop perspective. One can typically assume that the BS will utilize the reported CSI by the UE when scheduling a transmission. However, the BS doesn't consider if this CSI is old, and how to proceed in this case. The most straightforward approach is that if the PMI becomes old, based on some specific criteria such as the UE speed, the BS could utilize Tx diversity instead of CLSM to increase the chances of correct reception at the UE. However, utilizing Tx diversity might be too conservative i.e. in case the channel conditions are good, although the CSI became unreliable/outdated (not valid), and would lead to a lower peak rate than what a UE could have otherwise achieved. On the other hand, utilizing CLSM might be too aggressive, leading to reception errors and retransmissions, which would also decrease the bit rate. This is illustrated by FIG. 1, according to which the BS determines firstly if the UE has enough priority weight for being scheduled and when so being the case determining whether the reported CSI is reliable/valid or not. When the CSI is valid the BS then proceeds by selecting CLSM for transmission to the UE and if the CSI is determined not to be reliable/valid the BS may choose to transmit using Tx diversity.
In order to keep track of the channel variations, UEs regularly send wideband PMI reports on PUCCH, every T ms. The smaller T is, the more updated information the BS has. However, in order not to deplete the PUCCH capacity by excessively utilizing it for PMI feedback, T is typically configured to a value that is larger than the channel coherence time. As such, simply relying on the fact that a certain CSI report would be valid until the next CSI report is received does not hold, at least not for all UEs in the system. This is further illustrated by FIG. 2 in which the BS might choose a transmission scheme for a UE configured in CL MIMO from a time-domain perspective. Once a UE has high enough priority weight to be scheduled, e.g. a determined time period has elapsed making the UE highly prioritized, the BS examines the validity of this UE's PMI/RI information i.e. if the CSI is reported during time t+x.
As can be seen from the FIG. 2, a first event occurs at time t and represents a reception at the BS of a CSI report from the UE. The validity can be a function of e.g. the UE speed (i.e. high speed UEs have PMIs with shorter validity period compared to low speed UEs). A second event occurs at time t+x and represents the time where the previously received CSI report becomes outdated, thus unreliable to utilize when scheduling the UE and/or selecting for example its CL MIMO transmission mode as the case for LTE. In case the PMI/RI is valid that is only before time t+x since the last reporting, it will be used when deciding on a proper transmission scheme for that UE. Otherwise, i.e. the CSI was reported before a time t+T, the BS might revert to Tx diversity (or utilize CLSM with the outdated PMI/RI). Third is represented in the figure as a third event which occurs at time t+T and which represents the reception at the BS of an updated CSI report from the UE. One can deduce that the longer the invalidity period is, the more ‘wild guesses’ the BS has to do about proper transmit scheme selection. Keeping in mind that it is not desirable to decrease the period over which periodic CSI reporting happens, it is desirable to avoid the aforementioned limitation.
Concerning aperiodic CSI reporting on PUSCH, the UE can even append this information i.e. the CSI report, to its uplink data transmission, in case it has data to transmit in the uplink. However, the scheduling of a user for downlink is (in principle) independent of its uplink data reception and is based on the UE's downlink scheduling priority weight. As such, a UE is not guaranteed to be scheduled in downlink directly after it receives a CSI update on PUSCH, especially in the presence of several active users in the cell. What would accentuate such a limitation is the processing capability that typically means that in a practical system, only a limited number of scheduling entities can be scheduled every Transmission Time Interval (TTI).
In short, one main limitation is that the BS will at times possess unreliable/outdated CSI (e.g. PMI/RI) information for a CL-configured UE, leading the BS to making too aggressive/conservative choices.